1. Field of the Invention
The present invention relates to an evaporation source used in a vacuum deposition apparatus for forming an organic layer or a conductive layer.
2. Discussion of Related Art
An organic light emitting display device has a principle of emitting light by itself as a proper energy difference is generated in an organic thin film of a glass substrate when voltage is applied between an anode and a cathode, the glass substrate being coated with an anode layer, the organic thin film, and a cathode layer. In other words, an energy remaining when injected electrons and holes are re-combined is generated as light.
An Indum Tin Oxide (ITO) film having a small surface resistance and excellent transmittance may be used as the anode layer of the organic light emitting device. In order to enhance light emitting efficiency, a multi-layer structure of a hole injection layer (HIL), a hole transport layer (HTL), an emitting layer (EML), an electron transport layer (ETL), and an electron injection layer (EIL) may be used as the organic thin film, and a metal film such as a LiF-Al etc. may be used as the cathode layer. The organic material used in the organic thin film is Alq3, TPD, PBD, m-MTDATA, TCTA, etc., and a dopant is cumarine6, BczVBi etc.
The organic light emitting display device using the organic light emitting device has excellent features in view of a rapid response speed, a low power consumption, a high brightness, etc. and is further able to be made super light weight and super thin so that it has been spotlighted as a next generation display. However, mass-production equipment for the organic light emitting device has not been yet standardized so that there is an acute demand for development of the proper mass-production equipment.
The mass-production process of the organic light emitting device may be largely divided into three parts, a pre-process, a post-process and an encapsulation process. The pre-process is a process of forming an ITO thin film on a glass substrate mainly using a sputtering technique, and a mass-production process thereof has been already commercialized for a liquid crystal display apparatus. The encapsulation process is a sealing process of a device in order to improve a life time of the device, since the organic thin film is very vulnerable to the moisture and oxygen in the air. The post-process is a process of forming the organic thin film and the metal thin film on the substrate using an evaporation method in a high vacuum state. In the post-process, a vacuum deposition method forming a pixel patterning using a shadow mask on the substrate by evaporating organic material under a high vacuum atmosphere is mainly used.
As indispensable factors in mass-production of the organic light emitting device, it is very important how a high speed deposition is realized, maintaining a high vacuum during the post-process, and the detailed matters thereof are as follows:
First, in order to minimize reciprocal pollution between the multi-layer organic thin films, several independent chambers should be used and a large-area substrate of 370×470 nm or more should be utilizable. At this time, the dropping of the substrate and the dropping of the metal mask by means of thermal expansion should be maximally suppressed. Second, the alignment of the mask should be rapid and an in-situ of the mask covered with a film should be cleanable. Third, a TACT time of two minutes or less (time rendered in outputting a next sheet thereof after a sheet of substrate is output) should be realized by making the transfer of the substrate between chambers fast. Fourth, an evaporation source guaranteeing uniformity of the organic thin film that is most effective on the characteristics of the organic light emitting device to be below ±5%, and a deposition process should be realizable. At this time, it takes a lot of time for the high vacuum equipment to break vacuum to be restored again so that one time use capacity of the evaporation source should be maximized so that the vacuum of the high vacuum equipment can be maintained as long as possible for a smooth mass-production. In other words, the supply frequency of the exhausted organic material or metal material should be minimized.
As the evaporation source, an apparatus using a face-down deposition method that deposition material is put into a crucible to be heated and the material is evaporated into the substrate positioned in the upper to be deposited is mainly used. However, in the aforementioned evaporation source, heat loss occurs in the upper of the crucible by means of the opening portion of the upper of the crucible. Accordingly, as the deposition material evaporated from the internal of the crucible to move to a film forming substrate is condensed around the opening portion by means of the abrupt temperature change between the inner side and the outer side of the opening portion, a phenomenon that the opening portion is clogged may occur. If the opening portion is clogged, it may cause a problem that evaporation rates of the deposition process become unstable.
Also, when melting deposition material is used in the evaporation source, the deposition material is melted by the heat inside the crucible and portions of the deposition material may be splashed to the upper of the crucible from the evaporation surface of the deposition material. At this time, portions of the deposition material may be fixed around the opening portion of the upper of the evaporation source. In that case, as the opening portion is clogged with the deposition material, the evaporation rates of the deposition material become unstable so that it may cause a problem that it is difficult to secure reproducibility of a film thickness formed on the substrate.
Furthermore, if the interval between the evaporation source and the substrate is reduced to 100 mm or less when the organic material or the metal material is deposited on the large-area substrate, the deposition efficiency may be increased. However, heat generated in the evaporation source may cause thermal damage to the organic thin film and the shadow mask or bring drooping of the substrate. Such problems may be more serious in the case of a multi-point evaporation source or a linear evaporation source in which heat is generated from a large area than a point evaporation source. For example, in the case of a soda-lime glass having a thermal expansion coefficient of about 10 ppm, when a temperature rises to 25° C., expansion of about 250 μm occurs from glass of 1 m. In this case, a precision patterning of a pixel is impossible. Also, even in the case of EAGLE 2000 substrate (product of Samsung Corning Precision Glass) having relatively low thermal expansion coefficient of about 3.2 ppm, expansion of about 8 μm occurs under the same condition.
As described above, for mass-production of the organic light emitting display device, there is a demand for an evaporation source that can maximally block heat transferred to the shadow mask or the substrate side, increase one time receiving capacity of the deposition material, and be suitable for a rapid deposition process, while equalizing the thickness of the deposited film.